Method for manufacturing a rotor for an electric machine

ABSTRACT

An electrical machine includes a stator and a rotor. The stator has a central opening that is configured to receive the rotor. The rotor includes a generally cylindrical first section comprising a first material mounted on an axially extending shaft within the central opening. The rotor further includes a second section having a second material of a predetermined thickness that is plated integrally over at least a portion of the first section. The second material has a higher electrical conductivity relative to the first material.

BACKGROUND

The invention relates generally to an electrical machine, and moreparticularly to a rotor of an electrical machine, such as a motor and/orgenerator. Specifically, embodiments of the present technique provideimproved loss characteristics in an electrical machine.

Typically, an electrical machine includes a rotor and stator that areutilized to convert electrical power to mechanical power or vice versa.The stator generally includes a large number of laminations of amagnetic material that are stacked together. Within the center of thelaminations, a central opening is formed throughout the length of thestator. The rotor may also include magnetic materials that are mountedon a shaft, which is disposed within the central opening of the stator.

The stator may include coils or windings that form magnetic poles in thestator, while the rotor may also include windings or be a solid portionof magnetic material. For instance, the stator may include windings thatare located adjacent to the central opening of the stator, while therotor may include windings that are located on the outside of the rotoradjacent to the stator.

Generally, a time-varying magnetic field is capable of producing eddycurrents in electrically conductive materials, such as those from whichthe stator and rotor are formed. The magnetic metals usually exhibit arelatively high electrical resistivity. Because eddy current formationprimarily depends on electrical and magnetic properties of the rotorsurface, the magnetic fields produced around the rotor induce eddycurrents in the rotor. These eddy currents dissipate energy viaresistive heating in the rotor that increases the temperature of therotor. As a result, eddy currents reduce the efficiency of theelectrical apparatus and present thermal problems for the electricalapparatus. Further, the transient behaviors of the electrical machineare, in part, dictated by the rate at which induced eddy currents decayin the rotor surface. Thus, these eddy currents may impact the operationof the electrical machine.

To reduce the heat from the eddy currents, the electrical and magneticproperties of the rotor surface may be modified by attachments toaddress these problems. However, such attachments may not beadvantageous in electrical machines operating at high speeds, such asspeeds exceeding 10,000 revolutions per minute because the centrifugalforces and centrifugal stress imposed on such attachments by rotation ofthe rotor may cause the attachments to disintegrate from the rotor.

Accordingly, there exists a need for controlling rotor magnetic andelectrical properties, especially near the surface of the rotor, whileproviding a desirable mechanical strength for use in high-speedelectrical machines.

BRIEF DESCRIPTION

The present techniques accordingly provide a novel approach to obviatethe aforementioned problems. In one embodiment of the present techniquesan electrical machine having a stator and rotor is provided. The rotoris disposed within a central opening in the stator. The rotor includes agenerally cylindrical first section comprising a first material mountedon an axially extending shaft within the central opening. The rotorfurther includes a second section having a predetermined thickness andcomprising a second material plated integrally over at least a portionof the first section. The second material has a higher electricalconductivity relative to the first material.

In another aspect, a rotor for an electrical machine is provided. Therotor includes a generally cylindrical first portion comprising a firstmaterial. The rotor further includes a second portion comprising a layerof a second material plated integrally over the first portion. The layerhas predetermined thickness, and the second material has a higherelectrical conductivity relative to the first material.

In yet another aspect, a method for manufacturing a rotor is provided.The method includes fabricating a generally cylindrical first section ofa rotor from a magnetic material. The method further includes plating asecond section over at least a portion of the first section of therotor. The second section comprises an electrically conductive materialthat is applied at a predetermined radial thickness over the firstsection. The electrically conductive material is configured to exhibit ahigher electrical conductivity relative to the magnetic material.

In still another aspect, a method is provided for applying a conductivelayer over a rotor of an electrical machine. The method includesdetermining a thickness of the conductive layer to provide specificsurface properties based on operating parameters of the electricalmachine. The method further includes plating the conductive layer of anelectrically conductive material over a surface of the rotor based onthe calculated thickness, wherein the conductive layer is configured toexhibit a higher electrical conductivity than the rotor.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a partial perspective view of an electrical machine inaccordance with aspects of the present techniques;

FIG. 2 is a cross-sectional view of a stator and windingless rotorformed in accordance with one embodiment of the present techniques

FIG. 3 is a flowchart of an exemplary method for manufacturing the rotorof FIG. 2 in accordance with one embodiment of the present techniques

FIG. 4 is a cross-sectional of a stator and rotor having slots for fieldwindings formed in accordance with another embodiment of the presenttechniques;

FIG. 5 is a flowchart of an exemplary method for manufacturing the rotorof FIG. 4 in accordance with one embodiment of the present techniques;and

FIG. 6 is a front elevation cross-sectional view of rotor and a statorhaving a core clamping flange in accordance with an embodiment of thepresent technique.

DETAILED DESCRIPTION

The techniques discussed below involve plating of a conductive materialon the metal surface of the rotor in high-speed electrical machines. Thematerials plated on the rotor surface are configured to have controlledand specified thickness and electrical conductivity. These materialsimprove rotor surface properties that reduce eddy current losses on thepole faces in high speed electrical machines. Certain embodiments of thepresent techniques are described hereinafter referring generally toFIGS. 1-5.

Referring to FIG. 1, an electrical machine 10 is illustrated inaccordance with aspects of the present techniques. The electricalmachine 10 may include, for example, a motor or a generator. Asillustrated, the electrical machine 10 includes a stator 12 and rotor 14that are utilized for energy (or power) conversion. The stator 12 has acentral opening 16 extending along the length of the stator 12 thatsurrounds rotor 14. The stator 12 generally includes multiplelaminations (not shown) of at least one magnetic material that arestacked together. The rotor 14 comprises a generally cylindrical section18 mounted axially on a shaft 20. The cylindrical section 18 of therotor 14 is generally fabricated from a magnetic material, such asmagnetic steel. In certain embodiments, the shaft 20 may be formedintegral to the generally cylindrical section 18.

The electrical machine 10 may be adapted for low speed or high speedoperation. Low speed electrical machines are used, for example, aspropulsion motors in electric ship drives. In the low speed operation,the rotor speeds generally do not exceed a few hundred revolutions perminute (rpm). In contrast, in high-speed electrical machines, such asthose used as compact generators, the rotor speeds may exceed 10,000rpm. In one embodiment of a high speed electrical machine, such as ahigh-speed motor, the cylindrical magnetic section 18 of the rotor 14may be adapted to conduct electrical current to magnetize the rotor 14,which, when subject to the time-varying AC magnetic fields produced bythe stator windings, causes the rotor 14 to rotate.

Generally, eddy currents are not generated when the rotor spinssynchronously with the rotating magnetic field produced by the stator 12because, in such a case, each point on the rotor 14 experiences asubstantially constant magnetic field. However, in many applications,this synchronous operation of the rotor 14 with the stator 12 is notmaintained. For example, certain motor applications employ switchingdevices, such as thyristors or silicon-controlled rectifiers, tosynthesize AC power having a desired frequency and amplitude forapplication to the stator 12. The AC power synthesized by such switches,in addition to the desired fundamental frequency, also containssubstantial undesired harmonics. As known to those skilled in the art,harmonics represent sharp transitions of high frequencies in the currentor voltage waveforms. This exposes the stator 12 and rotor 14 tovariations in magnetic field at the harmonic frequencies, therebygenerating eddy currents that cause excessive heat of the surface of therotor 14.

In the stator 12, the eddy current losses are reduced because of therelatively thin, mutually insulated laminations of the magnetic metal.The thinness of each lamination, and its insulation from its neighbors,provides an extremely resistive path, thereby limiting the induced eddycurrents. However the use of such laminations is not advantageous inlimiting eddy current losses in the rotor 14 because the stress imposedby rotation of the rotor at high-speed operation may cause a laminatedstructure to fail.

In the rotor 14, the eddy currents produced from the non-synchronousoperation in the electrical machine 10 may be a result of high frequencyharmonics of stator excitation circuitry (not shown). The eddy currentsare generally localized to the surface of the rotor 14, which isreferred to as the skin effect. This localization of the eddy currentstends to confine high frequency power loss to the surface, leaving theinterior of the rotor 14 relatively loss-free. Hence, in accordance withthe present techniques, the surface of the cylindrical section 18 of therotor 14 is plated with a material having a high electrical conductivityrelative to that of the magnetic material. Because the amount of heatand power dissipation varies proportionally with the resistance of thematerial through which it passes, plating the rotor 14 with a highlyconductive material may reduce the amount of power dissipated by eddycurrents, while not presenting the problems with machine operation.

In other attachment methods, the electrical and magnetic properties ofthe rotor surface may be modified by enveloping the rotor in a highconductivity material, or by mechanically attaching higher conductivitymaterials to the rotor surface, such as high conductivity amortisseur(damper) bars or wedges. However, as mentioned earlier, such techniquesmay be undesirable in high-speed applications, as the centrifugal stressimposed on such attachments by rotation of the rotor at high speeds maycause them to disintegrate from the rotor because of the stressesinvolved with high speed operation. That is, the conductive coatings andmechanical attachments may not provide a bond that is strong enough towithstand the forces exerted by the high-speed operation. Unusual rotorshapes or curvatures may also make mechanical attachment difficult.

Further, other techniques, such as air-cooling of the rotor 14, may notbe available to reduce the heat produced by eddy currents in high-speedelectrical machines. For instance, air-cooling of the rotor 14 causesincreased air-friction on the rotor 14, which is disadvantageous forhigh-speed operation. Specifically, for a rotor operating at speeds ofabout 10,000 revolutions per minute (rpm), air friction leads toincreased windage losses that reduce the efficiency of the electricalmachine 10. In fact, high-speed electrical machines are sometimesrequired to operate in a substantial vacuum to reduce windage losses.Further, the electrical design of some machines may not allow the airgap between rotor and stator to be large enough for good air cooling.Hence, air-cooling of the rotor 14 may not be practical in suchapplications.

Accordingly, a highly conductive material may be attached to the surfaceof the rotor 14 to reduce the heat and inefficiencies. For instance, ahighly conductive material may be integrally plated onto the rotorsurface because the plating leads to a stronger bond between thecylindrical section 18 and the high-conductivity layer. This strongerbond is particularly advantageous in high-speed applications wherein ahigh bond-strength between the cylindrical section 18 and the highlyconductive layer is desirable to withstand centrifugal stresses imposedby high rotational speeds of the rotor 14. The conductive layer isdiscussed in greater detail below in FIG. 2.

FIG. 2 illustrates a front view of the electrical machine 10 of FIG. 1in accordance with aspects of the present techniques. As illustrated,the rotor 14 includes a conductive layer 26 of a highly conductivematerial disposed over the magnetic cylindrical section 18 to apredetermined thickness t₁. As used herein, the term conductivity shouldbe understood to refer to electrical conductivity. By “highlyconductive,” it is meant that the conductive layer 26 is adapted toexhibit a higher degree of electrical conductivity than the cylindricalsection 18. One example of a highly conductive material includes copper,which exhibits high electrical conductivity, desirable corrosionresistant properties and is relatively inexpensive. Further, copper alsoexhibits properties of high machinability and ductility (i.e. can besubject to bending and torsion without being subject to fatigue).Another example of a highly conductive material includes silver, whichexhibits a higher electrical conductivity than copper. However, silvertends to tarnish and offers relatively lower mechanical strength incomparison to copper. In certain embodiments, the conductive layer 26may comprise copper alloys, including, for example combinations ofcopper and silver, to achieve desired plating and adhesion properties,while maintaining balanced mechanical, electrical and thermalproperties.

In addition to the conductive layer 26, a bonding layer 28 may beutilized to provide improved adhesion properties thus yielding a highbond-strength. That is, the bonding layer 28 may be disposed between themagnetic cylindrical portion 18 and the conductive layer 26 to increasethe strength of the adhesion of the conductive layer 26 to the magneticcylindrical portion 18. In one embodiment, the bonding layer 28 mayinclude a layer of nickel that is plated on the surface of thecylindrical portion 18 to a predetermined thickness t₂. Beneficially,nickel provides a desired adhesion with copper, but exhibits a higherelectrical resistivity (hence lower conductivity) than copper. Thus, thebonding layer may be applied at a certain thickness, such as thicknessest₁ and t₂ of the conductive layer 26 and the bonding layer 28, based oncalculated values that optimize the surface properties of the rotor 16.Many such bonding layers are known in the art of plating and are usuallytermed “strike layers”.

To determine the thickness of the conductive layer 26 and the bondinglayer 28, various factors may be utilized to determine the optimalthickness. For instance, the thickness t₁ of the conductive layer 26 andthe thickness t₂ of the bonding layer 28 may be based on parameterscalculated from electromagnetic field theory, as well as specificparameters of the particular machine. The design parameters may include,for example, stator winding details (for e.g. number of phases, numberof coils of stator winding per phase, number of turns per coil, numberof stator slots, number of winding layers, number of parallel circuitsper phase, pitch ratio, etc.), various diameters (for e.g. statorwinding inner and outer diameters, airgap diameter, rotor surfacediameter, etc.), material properties of the stator and rotor (magneticand electrical properties at a given operating temperature), and statorwinding current wave shape (defined by its Fourier harmoniccomponents—amplitude and rotor speed based frequency). The thicknessest₁ and t₂ are computed to reduce the total rotor surface loss, whileproviding a desirable strength and adhesion properties. Further, thethicknesses t₁ and t₂ may depend on the nature of the process used forplating, such as electroplating, hot isostatic pressing and/orhigh-velocity powder spraying, which are discussed in greater detailbelow. In one specific embodiment, the thicknesses t₁ and t₂ may becomputed by an iterative method implemented by a computer program thataccepts design parameters of the electrical machine 10, rotor 14 and/orstator 12 as inputs to arrive at optimal values of the thicknesses t1and t2.

FIG. 3 is a flow chart illustrating an exemplary process 30 formanufacturing a windingless rotor for an electrical machine, such as theelectrical machine 10 of FIG. 1, in accordance with one embodiment ofthe present techniques. The process 30 begins at block 32. In block 32,a generally cylindrical portion of the rotor may be fabricated from amagnetic material, such as magnetic steel. This fabrication may includeforging a cylindrical shaped portion from a block of magnetic steel. Atblock 34, the optimal thicknesses of the highly conductive layer and/orthe bonding layer may be computed based on design parameters of theelectrical machine, as described above.

Once the thicknesses of the conductive layer and the bonding layer arecomputed, the bonding layer is applied over the surface of thecylindrical portion based on a calculated thickness of the bondinglayer, as shown in block 36 following the surface preparation of themagnetic rotor 18. The bonding layer may not be applied over the entirecylindrical portion, but may cover a portion of the rotor. Next, atblock 38, the conductive layer is applied over the bonding layer.Similar to the bonding layer, the conductive layer may not be appliedover the entire cylindrical portion, but may cover specific portions ofthe cylindrical section. The application of the bonding and conductivelayers in blocks 36 and 38 may be performed in a variety ofelectro-chemical and metallurgical bonding processes. For instance, inone embodiment, the conductive layer may be electroplated over thesurface of the cylindrical section. Electroplating is useful because itprovides a smooth surface finish of the conductive layer. This smoothsurface reduces windage losses in high-speed electrical machines.Further, the process of electroplating produces reduced porosity in theconductive layer 26, which results in higher electrical conductivity ofthe conductive layer. The electroplating process also provides controlover the plating rates to enable the conductive layer to be applied to aspecific thickness. Still further, the electroplating process may beconfigured to provide desirable adhesion properties that have a highbond-strength, which is advantageous for high-speed applications, asdiscussed above. These adhesion properties may also be enhanced bydisposing a bonding layer between the cylindrical portion and theconductive layer.

In addition to the electroplating process, it should be appreciated thatplating of the bonding and conductive layers over the cylindricalsection may also be performed via other electrochemical or metallurgicalbonding processes, such as electroless plating, hot isostatic pressingand high-velocity powder spraying, etc. Electroless plating is achemical plating method not requiring electricity; hot isostaticpressing is a process which consolidates metals using high pressures andtemperatures, usually starting with metallic powders; high velocitypowder spraying consolidates metallic powders into a coating by throwingthem against a metallic surface with a velocity high enough to partiallymelt them.

Once the conductive layer is applied to the bonding layer or cylindricalportion, the rotor may be assembled with a stator. As shown in block 40,the rotor may be assembled by axially aligning the rotor within thecentral opening of the stator. The assembly of the rotor and stator mayinclude other steps, such as coupling other control circuits to thestator and rotor and forming the frame around the assembled rotor andstator. Thus, other assembling steps may be performed to complete theelectrical machines assembly.

Similar to the techniques discussed above with regard to FIGS. 2 and 3,the plating of a conductive layer may also be utilized for rotors havingfield windings as illustrated in FIG. 4. FIG. 4 illustrates a front viewof an electrical machine 42 having a stator 44 and a rotor 46 disposedinside a longitudinal opening 48 in the center of the stator 44. Thestator 44 and rotor 46 may operate and function similar to the statorand rotor discussed above with regards to FIGS. 2 and 3.

However, the rotor 46 in this embodiment may be configured to supportwindings. For instance, the rotor 46 may include a generally cylindricalportion 52, which may be made of a magnetic material. The cylindricalportion 52 may have longitudinal slots 54 that are machined on thesurface of the cylindrical portion 52. The longitudinal slots 54 may beconfigured to support field windings 56. The field windings 56, whenenergized, create magnetic poles 58 and 60 on the surface of the rotor46.

Similar to the discussion above, the rotor 46 includes a conductivelayer 66 comprising a highly conductive material plated over the polefaces 58 and 60 as well as teeth 68 of conducting region 62 and 64. Asin the earlier embodiment, the conductive layer 66 has specifiedthickness t₃ that is configured to reduce losses from eddy currents inthe pole faces and the teeth 68 in the conducting regions. Also, abonding layer 70 may be disposed between the cylindrical portion 52 andthe conductive layer 66 to a specified thickness t₄ to improve adhesionproperties between the conductive layer 66 and the cylindrical section,as discussed above. The thicknesses t₃ and t₄ of the conductive layer 66and the bonding layer 70 may be determined based on design parameters ofthe electrical machine 42, similar to the discussion above.

FIG. 5 is a flow chart illustrating an exemplary process 72 formanufacturing a rotor with winding slots in accordance with anotherembodiment of the present techniques. In this embodiment, the generalmanufacturing process is similar to the discussion above regarding thewindingless rotor. As such, the blocks 74-80 are similar in principle toblocks 32-38 of FIG. 3.

However, unlike the above referenced process, the rotor for this processhas slots machined into the rotor for the windings. For instance, atblock 82, longitudinal slots are machined on the surface of the rotorafter plating the surface with a high-conductivity material. Plating isperformed prior to machining of the slots to yield a uniform thicknessof the high-conductivity material over the teeth and the pole faces,thus reducing windage losses at high-speed operations. The process 72then proceeds at block 84 by winding coils on the slots machined on therotor surface. Once the coils are in place, the rotor may be assembledinside the stator, as shown in block 86.

The above-described plating techniques are also helpful in reducinglosses in certain components within the stator of an electrical machine.An example of such a component is a stator core clamping flange. Thepresent technique may be advantageous in reducing eddy currents in thestator core clamping flange surface, while providing a relatively simplemechanical structure.

FIG. 6 illustrates an exemplary electrical machine 88 in accordance withthe aforementioned embodiment. The electrical machine 88 includes astator member 90 having a stator core 92 formed of a plurality oflaminations 94 of a magnetic material. The stator member 90 furtherincludes armature windings 96 coiled around the laminated stator core92. The stator core 92 is clamped on both sides (one side shown) by acore clamping flange 98. A rotor 100 is disposed within the stator core92 and may include field windings 102. Certain embodiments of the rotor100 may not include field windings as illustrated earlier. The rotor 100is separated from the stator core 92 by an airgap 104.

During operation of the electrical machine 88, magnetic flux 106 fromthe rotor 100 may be linked to the stator core 92 and the core clampingflange 98. Due to laminations 94 in the stator core 92, flux linked tothe stator core 92 does not produce significant eddy currents on thestator core 92. However, eddy currents may be formed on the surface ofthe core clamping flange 98, which leads to heating up of the flange 98.Traditionally, conductive shields have been used to shield the coreclamping flange 98 from eddy currents caused by the flux linked to it.However, such shields generally need to be hung away from the flange 98to allow for a cooling medium to pass between them, which make for acomplex mechanical structure. In accordance with the present technique,a conductive layer 108 is plated over the surface of the core clampingflange 98 via any of the electrochemical or metallurgical processesdiscussed above. In certain embodiments, the conductive layer 108 maycomprise, for example copper, or copper alloys as discussed earlier.Additionally, a bonding layer 110 may be disposed between the conductinglayer 108 and the core clamping flange 98, to achieve desirable adhesionproperties as discussed earlier. As discussed earlier, the conductivelayer 108 is adapted to reduce eddy current losses in the core clampingflange 98 caused by fundamental frequency and high frequency harmonics.In such embodiments, however, alternative cooling techniques need to beemployed, such as using a hollowed flange with forced fluid cooling.

Beneficially, the present techniques provide a cost effective process tocontrol surface properties of rotors and other components in high-speedelectrical machines. These techniques may reduce motor losses andoptimize transient behavior for the electrical machine. The techniquesdescribed are particularly advantageous because they enable surfaceproperties to be designed for reducing losses. Further, the techniquesprovide control over the thicknesses and conductivities of theconductive layers applied to the surface of the rotor and othercomponents. Thus, the present techniques improve the efficiency of theelectrical machine and reduce the negative influence of transientbehavior.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1.-17. (canceled)
 18. A method for manufacturing a rotor, comprising:fabricating a generally cylindrical first section of a rotor from amagnetic material; and plating a second section over at least a portionof the first section of the rotor, wherein the second section comprisesa electrically conductive material that is applied to a predeterminedradial thickness and wherein the electrically conductive material isconfigured to exhibit a higher electrical conductivity relative to themagnetic material.
 19. The method of claim 18, wherein fabricating thefirst section comprises forging the generally cylindrical first sectionfrom magnetic steel.
 20. The method of claim 18, wherein plating thesecond section comprises electroplating the second section over the atleast a portion of the first section.
 21. The method of claim 18,wherein plating the second section comprises pressing a hot isostaticover the at least a portion of the first section.
 22. The method ofclaim 18, wherein plating the second section comprises spraying ahigh-velocity powder over the at least a portion of the first section.23. The method of claim 18, wherein plating the second section compriseselectrolessly plating the second section over at least a portion of thefirst section.
 24. The method of claim 18, comprising disposing abonding layer between the first section and the second section, whereinthe bonding layer is configured to provide an adhesive surface tostrengthen the adhesive bond between the second section and the at leasta portion of the first section.
 25. The method of claim 18, furthercomprising: machining a plurality of slots in the rotor after platingthe second section on the first section; and winding one or more coilsin the plurality of slots.
 26. A method for applying a conductive layerover a rotor of an electrical machine comprising: determining athickness of the conductive layer to provide specific surface propertiesbased on operating parameters of the electrical machine; and plating theconductive layer of an electrically conductive material over a surfaceof the rotor based on the calculated thickness, wherein the electricallyconductive material exhibits a higher electrical conductivity than therotor.
 27. The method of claim 26, wherein the operating parameters ofthe electrical machine comprise at least one of the stator windingcharacteristics, geometries of a stator and the rotor, materialproperties of the surface layer and the rotor, stator winding currentshape, or any combinations thereof.
 28. The method of claim 26, furthercomprising determining a thickness of a bonding layer, wherein thebonding layer is configured to provide a specific adhesion between therotor and the conductive layer.
 29. The method of claim 28, furthercomprising plating the bonding layer to the determined thickness betweenthe rotor and the conductive layer.
 30. (canceled)
 31. The method ofclaim 26, wherein plating the conductive layer comprises electroplatingthe electrically conductive material over the surface of the rotor. 32.The method of claim 26, wherein plating the conductive layer comprisespressing a hot isostatic over the at least a portion of the surface ofthe rotor.
 33. The method of claim 26, wherein plating the conductivelayer comprises spraying a high-velocity powder over the at least aportion of the surface of the rotor.
 34. The method of claim 26, whereinplating the conductive layer comprises electrolessly plating theelectrically conductive material over the surface of the rotor.
 35. Amethod for manufacturing a rotor, comprising: electroplating a secondsection over at least a portion of a first section of the rotor, whereinthe second section comprises a electrically conductive material that isapplied to a predetermined radial thickness and wherein the electricallyconductive material is configured to exhibit a higher electricalconductivity relative to the magnetic material.
 36. The method of claim35, comprising disposing a bonding layer between the first section andthe second section, wherein the bonding layer is configured to providean adhesive surface to strengthen the adhesive bond between the secondsection and the at least a portion of the first section.
 37. The methodof claim 35, further comprising: machining a plurality of slots in therotor after plating the second section on the first section; and windingone or more coils in the plurality of slots.
 38. The method of claim 35,comprising fabricating the first section from magnetic steel.